hgbook
view en/ch02-tour-merge.xml @ 699:a17d6390a480
More fixes to chapters 1 and 2.
| author | Bryan O'Sullivan <bos@serpentine.com> |
|---|---|
| date | Sun May 03 20:27:09 2009 -0700 (2009-05-03) |
| parents | 29f0f79cf614 |
| children | d2aacc06e562 |
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1 <!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->
3 <chapter id="chap:tour-merge">
4 <?dbhtml filename="a-tour-of-mercurial-merging-work.html"?>
5 <title>A tour of Mercurial: merging work</title>
7 <para id="x_338">We've now covered cloning a repository, making changes in a
8 repository, and pulling or pushing changes from one repository
9 into another. Our next step is <emphasis>merging</emphasis>
10 changes from separate repositories.</para>
12 <sect1>
13 <title>Merging streams of work</title>
15 <para id="x_339">Merging is a fundamental part of working with a distributed
16 revision control tool. Here are a few cases in which the need
17 to merge work arises.</para>
18 <itemizedlist>
19 <listitem>
20 <para id="x_33a">Alice and Bob each have a personal copy of a
21 repository for a project they're collaborating on. Alice
22 fixes a bug in her repository; Bob adds a new feature in
23 his. They want the shared repository to contain both the
24 bug fix and the new feature.</para>
25 </listitem>
26 <listitem>
27 <para id="x_33b">Cynthia frequently works on several different
28 tasks for a single project at once, each safely isolated in
29 its own repository. Working this way means that she often
30 needs to merge one piece of her own work with
31 another.</para>
32 </listitem>
33 </itemizedlist>
35 <para id="x_33c">Because we need to merge often, Mercurial makes
36 the process easy. Let's walk through a merge. We'll begin by
37 cloning yet another repository (see how often they spring up?)
38 and making a change in it.</para>
40 &interaction.tour.merge.clone;
42 <para id="x_33d">We should now have two copies of
43 <filename>hello.c</filename> with different contents. The
44 histories of the two repositories have also diverged, as
45 illustrated in <xref
46 linkend="fig:tour-merge:sep-repos"/>. Here is a copy of our
47 file from one repository.</para>
49 &interaction.tour.merge.cat1;
51 <para>And here is our slightly different version from the other
52 repository.</para>
54 &interaction.tour.merge.cat2;
56 <figure id="fig:tour-merge:sep-repos">
57 <title>Divergent recent histories of the <filename
58 class="directory">my-hello</filename> and <filename
59 class="directory">my-new-hello</filename>
60 repositories</title>
61 <mediaobject>
62 <imageobject><imagedata fileref="figs/tour-merge-sep-repos.png"/></imageobject>
63 <textobject><phrase>XXX add text</phrase></textobject>
64 </mediaobject>
65 </figure>
67 <para id="x_33f">We already know that pulling changes from our <filename
68 class="directory">my-hello</filename> repository will have no
69 effect on the working directory.</para>
71 &interaction.tour.merge.pull;
73 <para id="x_340">However, the <command role="hg-cmd">hg pull</command>
74 command says something about <quote>heads</quote>.</para>
76 <sect2>
77 <title>Head changesets</title>
79 <para id="x_341">Remember that Mercurial records what the parent
80 of each change is. If a change has a parent, we call it a
81 child or descendant of the parent. A head is a change that
82 has no children. The tip revision is thus a head, because the
83 newest revision in a repository doesn't have any children.
84 There are times when a repository can contain more than one
85 head.</para>
87 <figure id="fig:tour-merge:pull">
88 <title>Repository contents after pulling from <filename
89 class="directory">my-hello</filename> into <filename
90 class="directory">my-new-hello</filename></title>
91 <mediaobject>
92 <imageobject>
93 <imagedata fileref="figs/tour-merge-pull.png"/>
94 </imageobject>
95 <textobject><phrase>XXX add text</phrase></textobject>
96 </mediaobject>
97 </figure>
99 <para id="x_343">In <xref linkend="fig:tour-merge:pull"/>, you can
100 see the effect of the pull from <filename
101 class="directory">my-hello</filename> into <filename
102 class="directory">my-new-hello</filename>. The history that
103 was already present in <filename
104 class="directory">my-new-hello</filename> is untouched, but
105 a new revision has been added. By referring to <xref
106 linkend="fig:tour-merge:sep-repos"/>, we can see that the
107 <emphasis>changeset ID</emphasis> remains the same in the new
108 repository, but the <emphasis>revision number</emphasis> has
109 changed. (This, incidentally, is a fine example of why it's
110 not safe to use revision numbers when discussing changesets.)
111 We can view the heads in a repository using the <command
112 role="hg-cmd">hg heads</command> command.</para>
114 &interaction.tour.merge.heads;
115 </sect2>
117 <sect2>
118 <title>Performing the merge</title>
120 <para id="x_344">What happens if we try to use the normal <command
121 role="hg-cmd">hg update</command> command to update to the
122 new tip?</para>
124 &interaction.tour.merge.update;
126 <para id="x_345">Mercurial is telling us that the <command
127 role="hg-cmd">hg update</command> command won't do a merge;
128 it won't update the working directory when it thinks we might
129 want to do a merge, unless we force it to do so.
130 (Incidentally, forcing the update with <command>hg update
131 -C</command> would revert any uncommitted changes in the
132 working directory.)</para>
134 <para>To start a merge between the two heads, we use the
135 <command role="hg-cmd">hg merge</command> command.</para>
137 &interaction.tour.merge.merge;
139 <para id="x_347">We resolve the contents of <filename>hello.c</filename>
141 This updates the working directory so that it
142 contains changes from <emphasis>both</emphasis> heads, which
143 is reflected in both the output of <command role="hg-cmd">hg
144 parents</command> and the contents of
145 <filename>hello.c</filename>.</para>
147 &interaction.tour.merge.parents;
148 </sect2>
149 <sect2>
150 <title>Committing the results of the merge</title>
152 <para id="x_348">Whenever we've done a merge, <command role="hg-cmd">hg
153 parents</command> will display two parents until we <command
154 role="hg-cmd">hg commit</command> the results of the
155 merge.</para>
157 &interaction.tour.merge.commit;
159 <para id="x_349">We now have a new tip revision; notice that it has
160 <emphasis>both</emphasis> of our former heads as its parents.
161 These are the same revisions that were previously displayed by
162 <command role="hg-cmd">hg parents</command>.</para>
164 &interaction.tour.merge.tip;
166 <para id="x_34a">In <xref
167 linkend="fig:tour-merge:merge"/>, you can see a
168 representation of what happens to the working directory during
169 the merge, and how this affects the repository when the commit
170 happens. During the merge, the working directory has two
171 parent changesets, and these become the parents of the new
172 changeset.</para>
174 <figure id="fig:tour-merge:merge">
175 <title>Working directory and repository during merge, and
176 following commit</title>
177 <mediaobject>
178 <imageobject>
179 <imagedata fileref="figs/tour-merge-merge.png"/>
180 </imageobject>
181 <textobject><phrase>XXX add text</phrase></textobject>
182 </mediaobject>
183 </figure>
185 <para id="x_69c">We sometimes talk about a merge having
186 <emphasis>sides</emphasis>: the left side is the first parent
187 in the output of <command role="hg-cmd">hg parents</command>,
188 and the right side is the second. If the working directory
189 was at e.g. revision 5 before we began a merge, that revision
190 will become the left side of the merge.</para>
191 </sect2>
192 </sect1>
194 <sect1>
195 <title>Merging conflicting changes</title>
197 <para id="x_34b">Most merges are simple affairs, but sometimes you'll find
198 yourself merging changes where each side modifies the same portions
199 of the same files. Unless both modifications are identical,
200 this results in a <emphasis>conflict</emphasis>, where you have
201 to decide how to reconcile the different changes into something
202 coherent.</para>
204 <figure id="fig:tour-merge:conflict">
205 <title>Conflicting changes to a document</title>
206 <mediaobject>
207 <imageobject><imagedata fileref="figs/tour-merge-conflict.png"/></imageobject>
208 <textobject><phrase>XXX add text</phrase></textobject>
209 </mediaobject>
210 </figure>
212 <para id="x_34d"><xref linkend="fig:tour-merge:conflict"/> illustrates
213 an instance of two conflicting changes to a document. We
214 started with a single version of the file; then we made some
215 changes; while someone else made different changes to the same
216 text. Our task in resolving the conflicting changes is to
217 decide what the file should look like.</para>
219 <para id="x_34e">Mercurial doesn't have a built-in facility for handling
220 conflicts. Instead, it runs an external program, usually one
221 that displays some kind of graphical conflict resolution
222 interface. By default, Mercurial tries to find one of several
223 different merging tools that are likely to be installed on your
224 system. It first tries a few fully automatic merging tools; if
225 these don't succeed (because the resolution process requires
226 human guidance) or aren't present, it tries a few
227 different graphical merging tools.</para>
229 <para id="x_34f">It's also possible to get Mercurial to run a
230 specific program or script, by setting the
231 <envar>HGMERGE</envar> environment variable to the name of your
232 preferred program.</para>
234 <sect2>
235 <title>Using a graphical merge tool</title>
237 <para id="x_350">My preferred graphical merge tool is
238 <command>kdiff3</command>, which I'll use to describe the
239 features that are common to graphical file merging tools. You
240 can see a screenshot of <command>kdiff3</command> in action in
241 <xref linkend="fig:tour-merge:kdiff3"/>. The kind of
242 merge it is performing is called a <emphasis>three-way
243 merge</emphasis>, because there are three different versions
244 of the file of interest to us. The tool thus splits the upper
245 portion of the window into three panes:</para>
246 <itemizedlist>
247 <listitem><para id="x_351">At the left is the <emphasis>base</emphasis>
248 version of the file, i.e. the most recent version from
249 which the two versions we're trying to merge are
250 descended.</para>
251 </listitem>
252 <listitem><para id="x_352">In the middle is <quote>our</quote> version of
253 the file, with the contents that we modified.</para>
254 </listitem>
255 <listitem><para id="x_353">On the right is <quote>their</quote> version
256 of the file, the one that from the changeset that we're
257 trying to merge with.</para>
258 </listitem></itemizedlist>
259 <para id="x_354">In the pane below these is the current
260 <emphasis>result</emphasis> of the merge. Our task is to
261 replace all of the red text, which indicates unresolved
262 conflicts, with some sensible merger of the
263 <quote>ours</quote> and <quote>theirs</quote> versions of the
264 file.</para>
266 <para id="x_355">All four of these panes are <emphasis>locked
267 together</emphasis>; if we scroll vertically or horizontally
268 in any of them, the others are updated to display the
269 corresponding sections of their respective files.</para>
271 <figure id="fig:tour-merge:kdiff3">
272 <title>Using <command>kdiff3</command> to merge versions of a
273 file</title>
274 <mediaobject>
275 <imageobject>
276 <imagedata width="100%" fileref="figs/kdiff3.png"/></imageobject>
277 <textobject>
278 <phrase>XXX add text</phrase>
279 </textobject>
280 </mediaobject>
281 </figure>
283 <para id="x_357">For each conflicting portion of the file, we can choose to
284 resolve the conflict using some combination of text from the
285 base version, ours, or theirs. We can also manually edit the
286 merged file at any time, in case we need to make further
287 modifications.</para>
289 <para id="x_358">There are <emphasis>many</emphasis> file merging tools
290 available, too many to cover here. They vary in which
291 platforms they are available for, and in their particular
292 strengths and weaknesses. Most are tuned for merging files
293 containing plain text, while a few are aimed at specialised
294 file formats (generally XML).</para>
296 </sect2>
297 <sect2>
298 <title>A worked example</title>
300 <para id="x_359">In this example, we will reproduce the file modification
301 history of <xref linkend="fig:tour-merge:conflict"/>
302 above. Let's begin by creating a repository with a base
303 version of our document.</para>
305 &interaction.tour-merge-conflict.wife;
307 <para id="x_35a">We'll clone the repository and make a change to the
308 file.</para>
310 &interaction.tour-merge-conflict.cousin;
312 <para id="x_35b">And another clone, to simulate someone else making a
313 change to the file. (This hints at the idea that it's not all
314 that unusual to merge with yourself when you isolate tasks in
315 separate repositories, and indeed to find and resolve
316 conflicts while doing so.)</para>
318 &interaction.tour-merge-conflict.son;
320 <para id="x_35c">Having created two
321 different versions of the file, we'll set up an environment
322 suitable for running our merge.</para>
324 &interaction.tour-merge-conflict.pull;
326 <para id="x_35d">In this example, I'll set
327 <envar>HGMERGE</envar> to tell Mercurial to use the
328 non-interactive <command>merge</command> command. This is
329 bundled with many Unix-like systems. (If you're following this
330 example on your computer, don't bother setting
331 <envar>HGMERGE</envar>. You'll get dropped into a GUI file
332 merge tool instead, which is much preferable.)</para>
334 &interaction.tour-merge-conflict.merge;
336 <para id="x_35f">Because <command>merge</command> can't resolve the
337 conflicting changes, it leaves <emphasis>merge
338 markers</emphasis> inside the file that has conflicts,
339 indicating which lines have conflicts, and whether they came
340 from our version of the file or theirs.</para>
342 <para id="x_360">Mercurial can tell from the way <command>merge</command>
343 exits that it wasn't able to merge successfully, so it tells
344 us what commands we'll need to run if we want to redo the
345 merging operation. This could be useful if, for example, we
346 were running a graphical merge tool and quit because we were
347 confused or realised we had made a mistake.</para>
349 <para id="x_361">If automatic or manual merges fail, there's nothing to
350 prevent us from <quote>fixing up</quote> the affected files
351 ourselves, and committing the results of our merge:</para>
353 &interaction.tour-merge-conflict.commit;
355 <note>
356 <title>Where is the <command>hg resolve</command> command?</title>
358 <para>The <command>hg resolve</command> command was introduced
359 in Mercurial 1.1, which was released in December 2008. If
360 you are using an older version of Mercurial (run <command>hg
361 version</command> to see), this command will not be
362 present. If your version of Mercurial is older than 1.1,
363 you should strongly consider upgrading to a newer version
364 before trying to tackle complicated merges.</para>
365 </note>
366 </sect2>
367 </sect1>
368 <sect1 id="sec:tour-merge:fetch">
369 <title>Simplifying the pull-merge-commit sequence</title>
371 <para id="x_362">The process of merging changes as outlined above is
372 straightforward, but requires running three commands in
373 sequence.</para>
374 <programlisting>hg pull -u
375 hg merge
376 hg commit -m 'Merged remote changes'</programlisting>
377 <para id="x_363">In the case of the final commit, you also need to enter a
378 commit message, which is almost always going to be a piece of
379 uninteresting <quote>boilerplate</quote> text.</para>
381 <para id="x_364">It would be nice to reduce the number of steps needed, if
382 this were possible. Indeed, Mercurial is distributed with an
383 extension called <literal role="hg-ext">fetch</literal> that
384 does just this.</para>
386 <para id="x_365">Mercurial provides a flexible extension mechanism that lets
387 people extend its functionality, while keeping the core of
388 Mercurial small and easy to deal with. Some extensions add new
389 commands that you can use from the command line, while others
390 work <quote>behind the scenes,</quote> for example adding
391 capabilities to Mercurial's built-in server mode.</para>
393 <para id="x_366">The <literal role="hg-ext">fetch</literal>
394 extension adds a new command called, not surprisingly, <command
395 role="hg-cmd">hg fetch</command>. This extension acts as a
396 combination of <command role="hg-cmd">hg pull -u</command>,
397 <command role="hg-cmd">hg merge</command> and <command
398 role="hg-cmd">hg commit</command>. It begins by pulling
399 changes from another repository into the current repository. If
400 it finds that the changes added a new head to the repository, it
401 updates to the new head, begins a merge, then (if the merge
402 succeeded) commits the result of the merge with an
403 automatically-generated commit message. If no new heads were
404 added, it updates the working directory to the new tip
405 changeset.</para>
407 <para id="x_367">Enabling the <literal
408 role="hg-ext">fetch</literal> extension is easy. Edit the
409 <filename role="special">.hgrc</filename> file in your home
410 directory, and either go to the <literal
411 role="rc-extensions">extensions</literal> section or create an
412 <literal role="rc-extensions">extensions</literal> section. Then
413 add a line that simply reads
414 <quote><literal>fetch=</literal></quote>.</para>
416 <programlisting>[extensions]
417 fetch =</programlisting>
419 <para id="x_368">(Normally, the right-hand side of the
420 <quote><literal>=</literal></quote> would indicate where to find
421 the extension, but since the <literal
422 role="hg-ext">fetch</literal> extension is in the standard
423 distribution, Mercurial knows where to search for it.)</para>
425 </sect1>
426 </chapter>
428 <!--
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430 sgml-parent-document: ("00book.xml" "book" "chapter")
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